Placebo, expectation, and the dopamine reward system

In our ongoing research into the modern wonder drug known as placebo, we stumbled across an interesting article at Endonurse.com summarizing a recent University of Michigan study on the subject.

Specifically, the research finds strong links between an individual’s response to a placebo "painkiller," and the activity of the neurotransmitter known as dopamine in the area of the brain known as the nucleus accumbens. That’s a small region at the center of the brain that’s involved in our ability to experience pleasure and reward, and even to become addicted to the "high" caused by illicit drugs.

The research team used detailed PET and fMRI brain scans to see how subjects reacted in the following situations.

The PET scans focused on brain dopamine, looking at its activity as volunteers were told to expect, and then received, a painful injection of saline solution in their jaw muscle. They were then told to expect, and then received, an injection that they were told could either be a painkiller or a placebo. (Both were in fact placebos.) The fMRI scans looked at volunteers’ brains while they played a game. Before each round, they learned that a correct answer would win or lose an amount of money, up to $5.

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Before the painful injection began, but after the volunteers had been told it was coming, they were also asked to guess how much pain relief they’d get from the "painkiller" if they received it.

The study shows that subject's anticipation of pain relief and expectation of reward (for being correct) was a good indicator of how strong the patient's dopamine response would be when receiving the placebo, and suggests that placebo response may be part of an inherent reward and self-preservation mechanism. The article goes into good detail at how the study was conducted and how results were screened for accuracy. Although the subject pool was small, this study (and another recent study on placebo and opiate response) goes a long way towards resolving the placebo mystery once and for all.